The purpose of the proposed research is to understand the mechanism by which Taxol manifests its effects on cells in vivo. We wish to test the hypothesis that microtubule-associated proteins (MAPs) are important participants in the cellular response to Taxol, either by contributing to MT stabilization in concert with Taxol, or by interacting with Taxol- stabilized MTs to produce deleterious consequences such as mitotic arrest. Taxol is known to target the microtubule (MT) system, causing MT stabilization that inhibits cell growth, disrupts intracellular organization, blocks cells in mitosis, and eventually leads to cell death. Therefore, MAPs that alter MT stability or are themselves altered in their interaction with stabilized MTs might be expected to either modulate Taxol-induced MT stabilization directly or to participate indirectly in downstream events. Variation in the presence, abundance, or regulation of these MAPs among human cell types would be expected to alter Taxol's cytotoxic potential. We have identified a candidate MAP, ensconsin, a 110kDa MAP that is present in a variety of human cells. Ensconsin shows unusually tenacious association with MTs both in vitro and in vivo in the presence of Taxol, and ensconsin accumulates along MTs and mitotic spindles in Taxol-treated cells. Our preliminary data raise the possibility that diverse levels or altered regulation of ensconsin may actually be correlated with disparate effects and potency of Taxol in several lines of human cells. We propose experiments aimed at understanding 1) the interaction of ensconsin with MTs in several clinically relevant cell types, 2) the cell's mechanism of regulation of ensconsin amount or function, and 3) the alterations in ensconsin that accompany Taxol administration, and the sequelae of these alterations, if any on Taxol's cytotoxic effects. The information gained from these studies will aid in our understanding of the mechanism by which Taxol acts on the MT system. In addition, our results may allow us to predict the efficacy of Taxol treatment of various tumor cells. Eventually, our studies of Taxol's mechanism of action may be beneficial in designing better strategies for the use of Taxol, or in designing improved Taxol-like drugs.